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Absorption and time resolved THz dynamics in liquid crystals

Changes of LC refractive index at the femtosecond time scale has been reported by inducing an ultrafast electronic Kerr response. Moreover, such an electronic excitation relaxed via coherent molecular vibrations modulating the LCs birefringence at a frequency of few THz [1].

The research proposed here focuses on the understanding of such low-frequency phenomena across LCs phase transition from isotropic (liquid) to liquid crystal and finally solid phase. In particular, this investigation will allow to define the degree of localization in collective molecular dynamics excited with phase-stable THz pulses, answering questions such as:

  • What is the nature of the collective molecular dynamics in LCs vs. single-molecular dynamics?
  • Can resonant collective dynamics transiently impact on LCs electronic and/or structural properties?
  • Which is the physical principle behind THz radiation absorption in LCs? Is it the same across the entire 1-10 THz spectral region?
  • Can we induce phonon-like excitation in liquid crystals?

A THz pump- optical probe set up has been built for this purpose, where high-field broadband THz radiation is obtained by pumping an organic crystal with an OPA, driven by a state-of-the-art amplified Ti:Sapphire laser system (~35 fs, ~9 mJ). The generated THz beam is further characterized by electro-optical sampling technique.

Recently we perform broadband THz time domain spectroscopy on 4-cyano-4′-alkyl-biphenyl (nCB) and 5-phenylcyclohexanes (PCH5) across mesophases. Density functional theory calculations on isolated molecules captured the majority of the response. In particular, the pronounced modes around 4.5 and 5.5 THz mainly originate from bending modes of the cyano group. In contrast, the broad response below 3 THz, linked to modes of the alkyl chain, disagrees with the single molecule calculation. Here, we identify a clear intermolecular character of the response, supported by dimer and trimer calculations. See our latest publication [2].

  1. L. Cattaneo, et al., Ultrafast all-optical response of a nematic liquid crystal. Optics Express, 2015. 23(11): p. 14010-14017.
  2. P. Friebel,et al. Unveiling low THz Dynamics of Liquid Crystals: Identification of Intermolecular Interaction among Intramolecular Modes. J. Phys. Chem. B 2024, 128, 2, 596–602. DOI: